A LED array, which comprises plural LEDs arranged along a straight line, is used as a printing head of an optical printer. As shown in FIG. 1, the conventional LED array is composed of plural LEDs 1, transistors 2 serving as drivers for on/off controlling turning-on currents supplied to individual LEDs, a driving voltage supply terminal (+) 4 for supplying a power supply voltage of positive polarity, a driving voltage supply terminal (-) 5 for supplying a power supply voltage of negative polarity, and turning-on signal input terminals 6 for supplying on/off controlling signals to the drivers.
An anode of the LED 1 is connected with the driving voltage supply terminal (+) 4, a cathode of the LED 1 is connected with a drain of the transistor 2, a source of the transistor 2 is connected with the driving voltage supply terminal (-) 5, and a gate of the transistor 2 is connected with the turning-on signal input terminal 6. The LED 1 can be turned-on by applying a predetermined voltage to the turning-on signal input terminal 6.
When one of the LEDs becomes faulty, this diode is not turned-on, even when the turning-on signal is applied to a driver of the LED, so that a corresponding dot cannot be printed. Whether the LED is turned-on (lighted) or not can be discriminated by examinating the state of lightings of the LEDs using an external photosensor, or inspecting the actual results of printing, but such a particular examination or inspection is troublesome. Accordingly, it is desirable to develop a LED array having a function of discriminating the existence of a faulty LED.
Heretofore, following technologies have been proposed as the ones for detecting a faulty diode.
In a LED driver circuit disclosed in Japanese Patent Kokai 1-238075, each LED is provided with a reference voltage-generating circuit, which generates a reference voltage for monitoring a forward voltage of the LED, and a comparator for comparing the forward voltage of the LED with the reference voltage; and an extraordinary signal is generated in case that the forward voltage of the LED exceeds the reference voltage because of an imperfection of an electrical contact. Furthermore, an OR circuit for deriving a logical sum of the outputs the comparators is provided in the LED driver circuit. However, this LED driver circuit has not a function of detecting an short circuited faulty LED. Moreover, since the reference voltage-generating circuit is built in the LED driver circuit, it is no easy matter to adjust the reference voltage from the outside of the LED driver. Accordingly, it is very difficult to change a reference voltage adjusted for detecting imperfection of an electrical contact in a LED circuit into that suited for detecting the state of a deteriorating LED, before it is thoroughly faulty. Moreover, since several thousands of LEDs are used in a LED array in an optical printer, it is actually impossible to fabricate an OR circuit for driving a logical sum of the outputs of several thousands of comparators.
In a faulty LED-detecting circuit for a LED array disclosed in Japanese Patent Kokai 2-128865, a resistor for detecting a current supplied to a LED is connected in series therewith. In an ordinary case (in case that an optical printer is operating), the resistor is bypassed by a relay, and in case that a faulty diode is searched for, the relay is open-circuited and a voltage applied to both the ends of the resistor is inspected. However, according to this faulty LED detecting circuit, the faulty LED cannot be detected, while the optical printer is operating. Moreover, since a single resistor is commonly used for all the LEDs, all the LEDs must be inspected in regular order one by one, and it is impossible to detect plural faults of the LEDs instantaneously. Furthermore, since the value of the resistance of the resistor is fixed, this faulty diode-detecting circuit can be applied only to the case that the driving currents of all the LEDs are the same. Furthermore, according to the faulty LED-detecting circuit mentioned in the above, the LED is regarded as faulty in case that the driving current flowing to the LED is less than a predetermined value, this circuit cannot detect the short-circuited faulty diode.
In a faulty diode-detecting circuit disclosed in Japanese Patent Kokai 5-31956, a common anode voltage is applied to plural LEDs to be inspected, and turning-on signals are applied to gates of switching transistors, which are respectively connected with cathodes of the LEDs, in regular order synchronizing with a certain clock signal. The sum of the anode currents of the respective LEDs are detected by means of a toroidal coil. In case that a faulty diode with an infinite impedance is driven, the toroidal coil does not detect an anode current, so that the existence of the faulty LED can be detected. However, according to the faulty diode-detecting circuit mentioned in the above, a short-circuited diode cannot be detected. Furthermore, this circuit cannot be applied, while an optical printer is operating.
In a LED array disclosed in Japanese Patent Kokai 5-212905, a voltage-detecting resistor is connected with data lines for respectively supplying constant currents to LEDs via diodes, and a voltage detected thereby is supplied to a window comparator. However, the resistance of the voltage-detecting resistor must be high so that the light-emitting characteristic of the LED is not changed. In case that the LED is short-circuited, since the potential of the data line becomes zero, a voltage applied to both the ends of the voltage-detecting resistor connected with the cathode of the diode becomes uncertain. Moreover, it is impossible to set a reference voltage of the window comparator from the outside thereof. Furthermore, in case that the plural LEDs connected with a common voltage-detecting resistor are turned-on at the same time, all the currents flowing through the LEDs meet one another in the voltage-detecting resistor and the voltage cannot be exactly measured.